Brain Fog: Causes, What Helps and When to Worry

Brain fog is a common group of symptoms that affect how you think, remember and concentrate. It is not a diagnosis in itself. [1]

Most people have a foggy day now and then. But if it keeps coming back, it is worth knowing what can cause it and what to tell your doctor. This guide covers what it feels like, the common causes, what helps and when to get help.

What brain fog feels like

People describe brain fog in different ways. Common signs are: [1] [6]

• losing your train of thought, or forgetting what you were about to say
• not finding the right word when you need it
• thinking and reacting more slowly than usual
• trouble concentrating or paying attention
• forgetting things you have just been told
• finding planning, organising or doing two things at once hard
• feeling mentally exhausted

It is different from being sleepy. You can feel awake and still struggle to follow a conversation or read a page.

Brain fog is common. A 2025 paper in BMC Public Health says more than a quarter of adults report having it. For some people it lasts a day or a week; for others it lasts much longer. [2]

What causes brain fog?

There is no single cause. Doctors don’t yet fully understand why it happens, and several things can add up at once. [1] [6] These are the common ones:

Cleveland Clinic also lists poor nutrition, ADHD and autism, and long hospital stays. One theory is that the immune system causes inflammation in the brain, which makes it harder to process information for a while. [1]

Why does brain fog come and go with chronic illness?

In energy-limiting conditions, brain fog often follows effort. The NHS lists problems with thinking, memory and concentration as one of the 4 main symptoms of ME/CFS. [3]

Post-exertional malaise (PEM) means symptoms get worse after activity. Mental activity counts too, such as studying or socialising. PEM can start hours or days after the activity, and it can take weeks to recover. [3] [4]

So if your fog turns up a day or two after a busy day, that delay is worth noting. Our guide to why you crash 24 hours after activity explains the pattern.

Being in pain, feeling very tired or feeling anxious can all make brain fog worse. [6] When several of these happen at once, there is rarely one fix.

Can thyroid problems cause brain fog?

Yes. The NHS lists difficulty concentrating or thinking clearly as a symptom of an underactive thyroid, along with tiredness, feeling cold, weight gain, constipation and low mood. [10] Hashimoto’s disease is a common cause.

A blood test for thyroid-stimulating hormone (TSH) and thyroxine (T4) shows whether your thyroid is underactive. It is usually treated with levothyroxine tablets, with regular blood tests to check the dose. [10]

If you take levothyroxine and the fog comes back, note when it started and whether your dose changed. Our Hashimoto’s symptoms checklist covers the other signs to look out for.

Is brain fog a symptom of MCAS?

It can be. Mast Cell Action, a UK charity, lists brain fog among the symptoms of mast cell activation syndrome, along with headaches and anxiety. [11]

MCAS symptoms tend to come and go. Most people have symptoms in several parts of the body at once, and flares can follow triggers such as infection, surgery or hormonal changes. [11]

If your fog comes with flushing, hives or stomach cramps, read our guide to MCAS symptoms.

What helps with brain fog?

There is no specific treatment for brain fog and no instant fix. [1] The best place to start is the cause, so it is worth seeing a doctor if it keeps happening. These everyday steps can make it easier to manage: [6]

• Get enough sleep and rest.
• Drink enough fluids and eat regular, balanced meals.
• Take regular breaks instead of pushing through.
• Use cues, key words and to-do lists to hold information for you.
• Write ideas down as soon as you have them, for example in a notes app.
• Slow down. Take your time and go back over information if you need to.
• Plan harder tasks for the time of day when you feel most alert.
• Notice patterns. Is the fog worse at certain times of day, or after certain activities?
• Ask about your medicines. If the fog started after a new medicine or a dose change, ask your doctor or pharmacist whether it could be a side effect.

If you have post-exertional malaise, treat thinking work like physical effort and pace it. Our energy pacing guide explains how, and spoon theory is a simple way to plan your day.

Exercise is sometimes suggested for brain fog, and lifestyle changes have helped some people, but results are far from consistent. [1] [2] If you have ME/CFS, don’t try to push through with more exercise. NICE, which writes guidance for the NHS in England, says people with ME/CFS should not be offered graded exercise therapy or any programme with fixed increases in activity. [7] Talk to your doctor before starting an exercise plan.

Is brain fog dangerous or permanent?

Usually not. Brain fog often lasts days to weeks, though in some cases it lasts months or years. [1] Most people with long COVID symptoms see a big improvement after 3 months, but some take much longer. [5]

Forgetting things now and then is common, and it is only occasionally a sign of something more serious, such as dementia. [12] Brain fog that rises and falls with sleep, effort or flares is the more usual pattern in chronic illness. Fog that keeps getting worse over weeks needs checking sooner.

When should you see a doctor about brain fog?

See a GP or doctor if:

• brain fog gets in the way of your daily life or work [1] [12]
• you keep having memory problems [12]
• it keeps getting worse over weeks instead of coming and going
• it started after a new medicine or a dose change [1]
• it comes with other new symptoms, such as tiredness that doesn’t go away, feeling cold, or dizziness when you stand up [8] [10]

What should you write down for your doctor?

Brain fog makes appointments hard, because the symptom gets in the way of explaining it. A few short notes are enough. Write down:

• when it started, and whether it came on suddenly or slowly
• how often it happens and how long each episode lasts
• what it stopped you doing, for example “I couldn’t finish a work email”
• what came before it: a busy day, a bad night, a period, an illness or a new medicine
• any other symptoms that came with it
• your medicines and any recent dose changes
• one question you want answered

Our guide to preparing for a medical appointment with brain fog has a simple plan, and the free appointment one-pager fits it all on one sheet.

If writing is too much on a foggy day, you can tell Juno by voice or text instead. Juno logs it for you and turns your notes into a dated summary to take to your doctor.

When is brain fog an emergency?

Brain fog that comes and goes is not an emergency on its own. Sudden confusion is different: it can be a sign of a stroke or another serious problem. [15]

This article is for general information only and does not replace advice from your own clinician. Juno is a health assistant, not a medical provider.

Last updated 17 October 2026